qt48: maintenance update, try enableParallelBuilding

I successfully built it with 8 free cores;
hopefully it won't give problems.

/cc maintainers: @lovek323, @phreedom, @sander.

I wonder that the build reports no support for gstreamer and pulseaudio.
I checked it was so before this commit, but it might still be useful
to enable those.

No more 4.8.x releases are planned. End of support is scheduled to Dec'15.